1. Field of the Invention
The present invention is concerned with an improved, low cost keyboard preferably formed of moldable synthetic resin material and which has a substantial degree of mechanical N-key rollover protection and other necessary features making the keyboard applicable for a wide variety of uses. More particularly, it is concerned with such a keyboard having a momentary impulse output operation, standard tactile feedback and the ability to handle high speed inputs without difficulty.
.[.Attention is directed to pending application for U.S. Letters Patent Ser. No. 06/190,177, Filed: Sept. 24, 1980; this application is hereby incorporated by reference into the instant application..].
.Iadd.Attention is directed to application Ser. No. 06/190,177, Filed Sept. 24, 1980, now U.S. Pat. No. 4,359,612. This patent is hereby incorporated by reference into the instant application. .Iaddend.
2. Description of the Prior Art
Keyboards are most commonly associated with typewriters and have until recent times developed in parallel with typewriter evolution. However, with the advent of the electronic age, a new generation of keyboards suitable for use as instruction keys for electronically activated devices has evolved. These keyboards have a wide array of uses, only one of which is to input electronic typewriters.
In the present state of the art, there are basically three types of keyboards. In one variety, electronic output in the form of electrically encoded signals to a companion or remote device is employed. In another type of keyboard, mechanical output movements are used which trip or activate leverages or linkage in either totally mechanical machines (e.g., manual typewriters) or electric machines such as electric typewriters. The principal distinguishing feature between these two types of keyboards is the form of output, i.e., mechanical movement or electric signal.
The third type of general keyboard construction can be thought of as a hybrid between the electronic and mechanical units. In this form, a mechanically induced movement is read electronically by one of various kinds of transducers, and the reader outputs the detected movement in the form of signals of an electronic nature.
While it is true that the keyboard art is old and well developed, the relatively recent proliferation of electronic devices that require operator instruction has caused the manufacturing of keyboards to grow at an enormous rate. Keyboards are required in all sizes, configurations, colors, shapes, tilts, slants, legends, codings, key strokes and depths. Despite this industry growth, or perhaps as a result thereof, no one keyboard or variety of keyboard has emerged as clearly superior. This is primarily due to the operational or cost limitations inherent in the various keyboard constructions, as well as the difficulty of modifying the same for particular purposes.
For example, in the case of full keystroke keyboards, the depth of the keyboard structure becomes a problem in many cases. The standard key switch plunger arrangement or mechanical leverage linkage consumes a considerable depth, because of the structural constraints encountered in developing a proper key stroke (approximately 3/16 of an inch) with acceptable key wiggle, direct depression travel, proper chassis and mounting cannister for either the key switch plunger or the leverage that attaches to the key stem. This in addition to the height of the key top itself necessitates a rather large, bulky overall keyboard structure. Because of the foregoing problems, full stroke keyboards are generally limited to conventional typewriters or input/output devices, and are not used on other types of equipment. Manifestly, the problem of providing a full stroke keyboard with minimum depth has limited the market potential of prior full stroke keyboards.
Virtually all known keyboards with key stroke capability require separate key tops. This is a fundamental requirement of the plunger or lever structure used as the key top support. The present industry standard for key tops on reliable equipment is double injection molded synthetic resin key tops. In this form, the key tops are first molded in one color of synthetic resin and an inner shell space is allowed for a second color injection that results in key legending being injected completely through the outer key shell. This process is inherently expensive in many ways. For example, it requires two complete injection runs for manufacture of the keytops, using expensive molding equipment which cannot be altered except at great expense. The double molding operation also results in a key top that is of substantial thickness and consumes a considerable amount of material.
The primary use of full stroke keyboards is in graphics and typewriting, including computer and CRT units. However, the bulk of the potential market for typewriter and printer equipment has grown accustomed to the tactile feel of conventional electric typewriters. These devices have feedback as a consequence of their design and mechanical construction. Tactile feedback in this context refers to a slight pressure increase required to depress a key through the initial range of key stroke, followed by a breakaway at about two-thirds of the stroke depth that is felt by the operator. This breakaway change from one pressure to a lighter pressure is not mimicked in any electronic keyboard in common usage, and accordingly this latter type of keyboard is deficient in this respect. In order to fully meet market demand and appeal to an already trained public, a keyboard form should include tactile feedback. Moreover, the amount of feedback should be variable without significant or costly manufacturing changes, in order to meet differing uses.
With touch typing at high speed on a conventional typewriter keyboard a phenomenon occurs which is referred to as "rollover." While typing at high speed, one key is in initial stages of depression before the priorly depressed key is released, and in some cases there could be as many as four keys simultaneously in various stages of depression, bottom out or upward travel. There are two ways to handle this problem that are in common use, i.e., mechanical blocking or filtering, or electronic scanning or logic analysis. Typical electric typewriters with keyboards use the technique of mechanical filtering. In this scheme some form of continuous chain of elements is configured in such a way that only one key lever at a time can pass through the chain. In this manner, no two keys can be in a position to interrupt or actuate a mechanical movement simultaneously. Because of the relatively high tolerance requirements of such systems, they are inherently expensive, can actually retard the speed of the typist, and present maintenance problems in that they can become gummed up and sticky over time. The typical electronic keyboard on the other hand solves the problem in an electronic way. Normally, a keyboard matrix of the key switch positions is scanned at high frequency. The first switch to be activated is entered into memory and the second switch is then entered while the output from the first switch is ignored or blocked and so forth until "N-keys" are depressed. For this reason electronic keyboards require a substantial amount of logic circuitry, the relative amount and sophistication of the decoding and "N-key" analysis and speed of information scanning being in direct proportion to the cost of the board.
Another absolute necessity in connection with keyboards is that of reliability, i.e., the life or number of cycles which can be expected from the keyboard, and within a given number of cycles, the number of misses or fault signals that occur. The most expensive and reliable keyboards on the market today are so-called "Hall effect" keyboards. In these units key depression closes a switch which is magnetically sensed, and only a breakdown in the mechanics of the switch cannister or chassis can effect reliability of such a device. However, Hall effect keyboards are inherently very expensive by virtue of the many electronic components required, and particularly the relatively high electronic power supply requirements.
In short, it will be appreciated that the various keyboards of the prior art each possess a number of outstanding attributes, but all are plagued by one or more serious deficiencies. Accordingly, there is a real and heretofore unsatisfied need in the art for a simple, low cost keyboard having the combined properties of full stroke capability, tactile feedback, N-key roll-over protection, minimum depth, and a high degree of reliability.